Effect of Active Structural Growth on Deepwater Reservoir Architecture: An Example from the Cretaceous Upper Gosau Subgroup, Northern Calcareous Alps, Austria

Morgan D. Sullivan¹, Donald A. Medwedeff², Jim Borer³, Hugo Ortner⁴, Julian Clark⁵, Douglas Paton³, Bruce Trudgill³, and Robert Amerman^3
1 Chevron Energy Technology Company, Houston, TX
² Basin Analysis Team, ChevronTexaco Energy Technology Company, San Ramon, CA
³ Colorado School of Mines, Golden, CO
⁴ University of Innsbruck, Innsbruck, Austria
⁵ Chevron Energy Technology Company, San Ramon, CA

To better understand the effects of active structural growth on deep-water reservoir architecture, an integrated study of the Upper Cretaceous deposits of the Upper Gosau Subgroup, Austria was initiated. The deep-water turbidites of the Upper Gosau Subgroup can be divided into three unconformity bounded sequences interpreted to be related to variations in rates of structural growth and deposition. The offlapping stratal geometries of Sequence 1 suggest that the rate of structural growth was greater than the rate of deposition. The progressive onlapping stratal geometries of Sequence 2, however, suggest that the rate of deposition was greater than the rate of structural growth. Due to limited preservation of Sequence 3, its stratal geometries are less clear. Dominating all three sequences of the Upper Gosau Subgroup are vertically-stacked, upward-fining, laterally continuous, unchannelized conglomerates and sandstones that display little to no lateral variation in facies. These deposits are interpreted as laterally confined sheets due to the lack of lateral changes in facies (i.e., the flows were as large as the width of the basin). The distinct vertical changes in facies, however, suggest that these flows were not axially confined (i.e., the flows were smaller than the length of the basin). This observation is consistent with previous work in confined deep-water turbidite systems and suggests that this type of depositional setting is commonly dominated by laterally continuous, confined sheets. This is in distinct contrast to the architecture of deep-water turbidites deposited in unconfined settings which typically exhibit complex varieties of architectural styles.